(1) Field of the Invention
The present invention relates to delivery of contents via a network.
(2) Description of the Related Arts
Along with the improvement of terminal performance and the capacity increases of network lines and of hard disks, in recent years, content delivery systems have been made up. The content delivery systems deliver contents, including such kinds of data as image, video, audio, text, program, and explanatory information (meta-data), from a server to client terminals over an Internet Protocol (IP) network, thereby to enable users to watch and listen to the received contents at their terminals. IP television (IPTV) service for delivering such audiovisual works as television programs and movies by using this kind of content delivery has been developed.
Japanese unexamined patent publication No. 2005-135140 (Patent document 1) discloses an invention to accomplish an object of providing a technique with which a client who requested content can receive requested content delivery from a specific client having superior delivery quality and licensing conditions (See, paragraph [0012] of the Patent document 1) by sending from a server to the requesting client a list of clients to which the requesting client is connectable, so that the requesting client can select the optimum one in accordance with the delivery quality and licensing conditions out of the listed clients and receive content delivery from the selected client (See, paragraph [0013] of the Patent document 1).
Japanese unexamined patent publication No. 2006-72432 (Patent document 2) discloses an invention to accomplish an object of providing a peer-to-peer type content delivery system capable of balancing loads on the participating piers by providing a large number of dynamic peers DP and a center server CS for managing content delivery. Each dynamic peer DP notifies the center server CS of its own operating state, and the center server CS calculates and registers the load on each dynamic peer DP on the basis of the notified operating state. Each dynamic peer DP obtains a list of contents from the center server CS to search for the other dynamic peers DP each having the content it desires, and downloads the content from the least loaded dynamic peer DP. The system is further provided with a static peer SP for uploading a new content first. The content is delivered to dynamic peers DPs starting from the static peer SP (See, abstract of Patent document 2).
IPTV service is broadly classified into three types of service: streaming, download and progressive download.
In the streaming, a server delivers content data successively to a client, and the client represents video and audio from arrived data to present them to its user. This enables the user to watch and listen to the content on a substantially real time basis if a network has a sufficiently wide bandwidth.
In download, the client acquires all the content data from the server in advance, and starts representing of the content data for watching and listening after completing the storing of received content data. Because all the content data are delivered and stored in advance, the download type service enables the user to watch and listen to the content whenever and as many times as the user wants in the case where there is no need to watch and listen on a real time basis and to receive content delivery even if the network does not have a wide enough bandwidth.
Progressive download is positioning between the streaming type and the download type. In the progressive download, watching and listening of content are performed successively from content data having been stored in the terminal before completing the delivery of all the contents. The progressive download has an advantage that the user does not have to wait for the completion of all content data storing, the time for storing the content data is shortened substantially even if the bandwidth is not wide enough and the user can watch and listen to the content whenever and as many times as the user wants after completing the content data storing.
Commercial IPTV services, especially download-oriented services, are usually provided by a so-called server-client type system in which contents are distributed from a center server. In this system, content delivery is started when a user designates a desired content from among content lists offered by the center server.
In this system, for example, a part of the title, a text indicating the name of performer, genre attributes of the content, etc. that are designated by the user, are sent to the server. The server searches for the desired content by using the function of narrowing down the possible candidates according to a suitable content list.
As the content service provider has to take charge of many administrative processing, such as authentication of a terminal and a user that receive the content delivery as legitimate equipment and an authorized subscriber, charge accounting for each user who has contracted the content delivery service to play the content, managing the key information for enciphering content data and decrypting encrypted data to prevent unfair watching/listening of the content, etc., the server-client type is generally considered suitable for the content delivery service described above.
On the other hand, there is a peer-to-peer (P2P) type delivery as one of file sharing type delivery for delivering general contents by using PCs and the Internet. In this type of delivery, a terminal which is referred to as a peer or a client operates as a server to another terminal, and contents are delivered from one terminal to another terminal, namely from peer to peer. In this type, content delivery takes place in geometrical progression as a plurality of terminals perform successively the role of the server, unlike in the client-server type delivery in which a specified server carries out all the content delivery.
P2P type delivery is classified into a pure P2P type and a hybrid P2P type. In the pure P2P type, since every communication is carried out basically by peers with no dependence on the center server, the network has high tolerance to faults. It is also characterized by the capability to permit anonymity in mounting and a high efficiency of delivery.
Meanwhile, in the hybrid P2P type, content forwarding is carried out between peers, but resolution of content location and assignment of peers are carried out by the center server. That is, as the searching of content and peer having the content is performed by the center server and actual content delivery which needs a heavy processing load is carried out by peers, the hybrid P2P is a rational content delivery method permitting easy management of all terminal operations.
In considering these factors, techniques for spreading the IPTV service offering managed content delivery have been developed in the IPTV download delivery service as well by using a P2P type content delivery method, especially, by using a hybrid P2P type content delivery method.
For instance, in a hybrid P2P type contents delivery system according to Patent document 1, each peer is available content redelivery from a peer having good delivery quality and adequate licensing conditions with a long validity period. This type ensures stable delivery quality and licensing conditions because each client can receive content delivery from a peer having the best connection environment from among a plurality of peers.
According to Patent document 2, P2P type content delivery is realized by making each peer notify the center server of its own operation state so that the center server can select the least loaded peer.